1. Field of the Invention
The present invention relates to feedthrough assemblies for electrical or optical interconnection to the inside of a housing in sealed fashion, and more particularly to a feedthrough assembly in which a ceramic feedthrough is mounted within and brazed to an opening in a metal housing to provide a hermetic seal.
2. History of the Prior Art
It is known to provide feedthrough assemblies in which a feedthrough is mounted within an opening in a housing so as to extend from the outside to the inside of the housing. The feedthrough may be used to provide electrical or optical interconnection to the inside of the housing. An interface or joint between the feedthrough and the housing is typically brazed to provide the housing package with hermeticity.
In feedthrough assemblies of this type, the feedthrough is typically made of ceramic and the housing is typically made of a metal such as a nickel alloy. Brazing is typically carried out using a brazing compound comprised of a metal or mixture of metals. Because there is a substantial difference in the coefficient of thermal expansion between the ceramic of the feedthrough and the metal of the housing, the brazing operation often results in deformation or cracking of the material with the lower flexural strength. This is particularly true in the case of brazing compound materials such as those having a mixture of silver and copper which must be brazed at high temperatures of typically at least 780xc2x0 C. or more. Because of the problems attendant with such high temperature brazing, lower temperature brazing must often be used. For example, a gold and tin mixture can be brazed at approximately 300xc2x0 C. This minimizes the effects of the thermal mismatch between the ceramic and the metal, but at the expense of lower yield and higher cost.
To facilitate the brazing operation at both high and low temperatures, it is desirable to provide feedthrough assemblies in which the tight dimensional requirements of the ceramic-metal interface are relaxed. Desirably, the total overall dimensions of the ceramic feedthrough are kept to a minimum. However, this is not always possible, such as in applications where larger feedthroughs must be used. Certain assembly designs reduce the contact area between the ceramic feedthrough and the metal wall to a limited extent, thereby facilitating the brazing operation.
Nevertheless, conventional feedthrough assemblies are lacking in their ability to significantly relax the interface tolerance requirements and significantly reduce the surface area of contact between the feedthrough and the housings, so as to make the brazing operation significantly easier and more trouble-free. In particular, it is desirable that the assembly designs facilitate greater application of high temperature brazing.
Feedthrough assemblies in accordance with the invention reduce the requirement for close tolerances at the feedthrough-housing interface and facilitate the use of high temperature brazing, where desired, without the usual attendant problems such as deformation or cracking resulting from the thermal mismatch between the ceramic and the metal. This is accomplished in accordance with the invention by minimizing the surface area of contact between the ceramic feedthrough and the metal housing, particularly in the region of the lower surface of the feedthrough and the portion of the generally planar base of the housing which typically extends thereunder. In feedthrough assemblies according to the invention, the base of the housing is provided with an opening beneath the feedthrough. The opening preferably extends through an area which is not substantially smaller in size than the bottom of the feedthrough, and may extend under the opposite edges of the feedthrough by a small distance so as to define ledges thereunder. At the same time, the surface area of the interface between the base of the housing and the feedthrough is greatly minimized, and with it the thermal mismatch problems which accompany the operation when the feedthrough is brazed in place within the housing. Alternatively, the opening in the base may be made approximately equal in size to the feedthrough. In such arrangements, the edges of the opening may frame and abut the back and opposite side walls of the feedthrough so as to minimize the surface area of contact therebetween even further.
Feedthrough assemblies in accordance with the invention also eliminate the tight dimensional requirements of prior art arrangements by way of a cut-out design that allows the feedthrough to be placed through a corner of the housing rather than through a side of the housing. The side wall of the housing is mounted on the base thereof so as to have an opening at a corner of the base. The feedthrough is formed so as to be generally L-shaped in configuration and so as to reside within the opening in the side wall. The L-shaped feedthrough is comprised of two leg portions which are joined at a generally right angle at the corner of the base plate and which include a pair of lead frames extending from the two leg portions to the outside of the housing. This permits a large feedthrough at both sides of the housing. Again, the base of the housing is provided with an opening beneath the feedthrough, and the opening either extends to positions close to or at the back and side surfaces of the feedthrough, so as to minimize the surface area of contact between the housing and the feedthrough. The opening may either form small ledges in the base beneath the side surfaces of the feedthrough, or it may be generally coextensive with the feedthrough so that the edges of the opening abut the side walls of the feedthrough.
In a still further embodiment of a feedthrough assembly according to the invention, the base of the housing is provided with a slot in the underside thereof which extends into the base from a side edge thereof. The slot extends upwardly through part but not all of the thickness of the base. A feedthrough is mounted within the slot in the base. The base has an opening therein adjacent the feedthrough which extends from an upper surface of the slot through the remainder of the thickness of the base, to substantially reduce the surface area of contact between the feedthrough and the housing and to provide access to the feedthrough from inside the housing. The slot, the feedthrough and the opening may be generally rectangular in shape, and the slot and the feedthrough are similar in size with the opening being slightly smaller.
The feedthrough is typically made of ceramic and the housing is made of a metal such as a nickel alloy. In spite of the substantial differences in the temperature coefficients of the ceramic and metal, brazing can be successfully accomplished using a high temperature braze such as that required for a brazing mixture of silver and copper. Again, the designs of the feedthrough assembly with their minimum area of contact between the ceramic and the metal permit the use of such high temperature brazing.